1. Field of the Invention
This invention relates to lubricating oil compositions and has particular reference to such a lubricating oil composition which has high oxidative stability and sludge-preventing performance characteristics.
2. Prior Art
For the purpose of providing improved oxidation stability of lubricants, it has heretofore been proposed to blend the starting base oil with an oxidation inhibitor such as a pheno-based compound, typically 2,6-di-t-butyl-p-cresol, and an amine-based compound, typically phenyl-.alpha.-naphthyl amine and alkyldiphenyl amine. However, it has been found that the 2,6-di-t-butyl-p-cresol is apt to decline in its oxidation inhibiting performance under elevated temperature conditions, and that the phenyl-.alpha.-naphthyl amine, though effective at high temperature, is less compatible with a lubricant base oil and susceptible to self-deterioration with oxidation, resulting in the formation of a sludge which in turn plugs up the filters in the lubricant supply circuit, or deposits on the heat-exchangers, and further that the alkyldiphenyl amine is likewise susceptible to sludge formation upon oxidation and inferior in high temperature performance to the phenyl-.alpha.-naphthyl amine.
The present inventors have previously proposed, as disclosed in Japanese Laid-Open Patent Publication No. 62-181396, to use a p-branched alkylphenyl-.alpha.-naphthyl amine derived from a propylene oligomer and have further proposed, as disclosed in Japanese Laid-Open Patent Publication No. 3-95297, to provide a lubricant composition comprising the aforesaid naphthyl amine (derived from a propylene oligomer) in combination with a p,p'-dialkyldiphenyl amine derived from a propylene oligomer.
Japanese Laid-Open Patent Publication No. 5-179275 discloses blending the above lubricant composition with a small amount of a hindered phenolic compound.
The foregoing prior lubricants are not fully capable of meeting the current stringent lubrication requirements for machineries and tools that are growing more compact and longer serviceable with higher output. A demand is acknowledgeable for high oxidation inhibitive lubricants capable of use in gas turbines, compressors, hydraulically actuated machines and the like that operate at extremely high temperatures and need protection against adverse effects of sludge.